Damping circuit for an energy storage device and method for damping oscillations of the output current of an energy storage device

ABSTRACT

A damping circuit for an energy storage device. The damping circuit comprises a current detection device designed to detect an output current of energy supply strings or the energy storage device and to generate an output current signal dependent on the output current. The damping circuit also includes a closed-loop control circuit coupled to the current detection device. The closed-loop control circuit designed to adjust the output current signal to a setpoint current signal and to output a corresponding current control signal. A first winding of a transformer is coupled to an output connection of the energy storage device. A second winding is galvanically isolated from the first winding. A compensation current generation device is coupled to the closed-loop control circuit, and is designed to feed a compensation current into the second winding of the transformer depending on the current control signal.

BACKGROUND OF THE INVENTION

The invention relates to a damping circuit for an energy storage deviceand to a method for damping oscillations of the output current of anenergy storage device, in particular for battery converter circuits forsupplying voltage as a variable current source for loads occurring suchas, for example, electric machines in drive systems of electricallyoperated watercraft or land vehicles.

It would appear that, in the future, electronic systems which combinenovel energy storage technologies with electric drive technology will beused increasingly both in stationary applications, such as wind turbinesor solar power installations, for example, and in vehicles such ashybrid or electric vehicles.

In order to feed alternating current into an electric machine, a DCvoltage provided by a DC voltage intermediate circuit is conventionallyconverted into a three-phase AC voltage via a converter in the form of apulse-controlled inverter. The DC voltage intermediate circuit is fed bya string of battery modules connected in series. In order to be able tomeet the requirements placed on power and energy which are provided fora specific application, a plurality of battery modules are oftenconnected in series in a traction battery. Such an energy storage systemis often used, for example, in electrically operated vehicles.

A series circuit comprising a plurality of battery modules is associatedwith the problem that the entire string fails when a single batterymodule fails. Such a failure of the energy supply string can result infailure of the entire system. Furthermore, temporarily or permanentlyoccurring reductions in power of a single battery module can result inreductions in power in the entire energy supply string.

Documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclosebattery cells connected to one another in modular fashion in energystorage devices which can be coupled into or decoupled from the stringof battery cells connected to one another in series selectively viasuitable actuation of coupling units. Systems of this type are knownunder the name Battery Direct Converter (BDC). Such systems comprise DCsources in an energy storage module string which can be connected to aDC voltage intermediate circuit for supplying electrical energy to anelectric machine or an electrical power supply system via apulse-controlled inverter.

BDCs generally have increased efficiency and increased fail safety incomparison with conventional systems. The fail safety is ensured, interalia, by virtue of the fact that defective, failed or incorrectlyfunctioning battery cells can be disconnected from the energy supplystrings by suitable bridging activation of the coupling units.

The energy storage module strings in this case have a plurality ofenergy storage modules connected in series, wherein each energy storagemodule has at least one battery cell and an associated controllablecoupling unit, which makes it possible to bridge the respectivelyassociated at least one battery cell or to connect the respectivelyassociated at least one battery cell into the respective energy storagemodule string depending on control signals. Optionally, the couplingunit can be designed in such a way that it additionally makes itpossible to connect the respectively associated at least one batterycell into the respective energy storage module string even with inversepolarity or else to interrupt the respective energy storage modulestring.

The total output voltage of BDCs is determined by the actuation state ofthe coupling unit and can be set gradually, wherein the graduation ofthe total output voltage is dependent on the individual voltages of theenergy storage modules. Owing to intrinsic complex impedances of theenergy storage modules and their components, the energy storage deviceacts with a downstream intermediate circuit capacitor as a resonantcircuit. The resonant frequency of this resonant circuit can vary withthe number, which differs depending on the voltage requirement, and theclock-pulse rate of the connected energy storage modules. This meansthat, when the energy storage device is coupled to a load acting asvariable current source, such as an inverter and an electric machineconnected downstream of the inverter, for example, undesired resonancescan occur.

There is therefore a need for measures with which the occurrence of suchresonances or current fluctuations during coupling of a BDC to anintermediate circuit capacitor for feeding a load acting as variablecurrent source can be reduced or suppressed.

SUMMARY OF THE INVENTION

In accordance with one embodiment, the present invention provides adamping circuit for an energy storage device, which has one or moreenergy storage modules which are connected in series in one or moreenergy storage strings, said energy storage modules comprising at leastone energy storage cell and one coupling device with a multiplicity ofcoupling elements, which coupling device is designed to selectivelyconnect the energy storage cell into the respective energy supply stringor to bridge said energy storage cell. The damping circuit comprises acurrent detection device, which is designed to detect an output currentof the energy supply strings or the energy storage device and togenerate an output current signal dependent on the output current, aclosed-loop control circuit, which is coupled to the current detectiondevice and is designed to adjust the output current signal to a setpointcurrent signal and to output a corresponding current control signal, atransformer, which has a first winding, which is coupled to an outputconnection of the energy storage device, and a second winding which isgalvanically isolated from the first winding, and a compensation currentgeneration device, which is coupled to the closed-loop control circuitand is designed to feed a compensation current, which compensates forfluctuations in the output current of the energy storage device, intothe second winding of the transformer depending on the current controlsignal.

In accordance with a further embodiment, the present invention providesa system, comprising an energy storage device, which has one or moreenergy storage modules, which are connected in series in one or moreenergy supply strings, said energy storage modules comprising at leastone energy storage cell and a coupling device having a multiplicity ofcoupling elements, which coupling device is designed to connect theenergy storage cell selectively into the respective energy supply stringor to bridge said energy storage cell, and a damping circuit accordingto the invention.

In accordance with a further embodiment, the present invention providesa method for damping oscillations of the output current of an energystorage device, which has one or more energy storage modules, which areconnected in series in one or more energy supply strings, said energystorage modules comprising at least one energy storage cell and acoupling device having a multiplicity of coupling elements, whichcoupling device is designed to connect the energy storage cellselectively into the respective energy supply string or to bridge saidenergy storage cell. The method comprises the steps of detecting anoutput current of the energy supply strings or the energy storagedevice, generating an output current signal which is dependent on thedetected output current, applying closed-loop control to the outputcurrent signal to adjust it to a setpoint current signal, outputting acurrent control signal corresponding to the closed-loop control,generating a compensation current, which compensates for fluctuations inthe output current of the energy storage device, and feeding thecompensation current into the second winding of a transformer, which hasa first winding, which is coupled to an output connection of the energystorage device, and a second winding, which is galvanically isolatedfrom the first winding.

ADVANTAGES OF THE INVENTION

One concept of the present invention consists in superimposing acompensation current on an output current of an energy storage devicewith modular energy supply strings comprising a series circuit of energystorage modules, with the result that resonances as a result of currentfluctuations of a variable current source fed by the energy storagedevice are damped. In this case, output currents of the energy storagedevice are detected and compensation currents are actively generateddepending on the output currents detected or the fluctuations in theoutput currents, wherein the compensation currents can be fed to thevariable current source via galvanic coupling together with the actualoutput currents of the energy storage device. The compensation currentsmatched flexibly in terms of their amplitude in this case serve toactively damp possible resonances.

As a result of this damping, on-board power supply system resonances canbe damped without any additional losses in the power path. By virtue ofinherent closed-loop control via the current feedback loop, it isadvantageously possible to match the damping to the position of theresonance frequency and the magnification factor of the resonantcircuit, irrespective of the instantaneous operating state of the energystorage device. In particular, measurement of the resonant frequency ofthe energy storage device is no longer necessary. Changes which canoccur, for example, owing to aging or changes in the system topology,for example owing to longer high-voltage lines, during operation of theenergy storage device are also automatically and flexibly compensatedfor by the damping circuit.

By virtue of the active closed-loop control, a degree of damping canadvantageously be selected by an adjustable frequency range. The energystorage device can in this case be operated in the conventional mannerwithout load current-conducting components needing to be clockedvariably in a manner subject to losses.

The damping circuit can advantageously be implemented completely in thelow-voltage range by the galvanic decoupling, as a result of which softswitching and a single feed from a 12 volt power supply system ispossible. The design and control parameters for the damping circuit arein this case independent of the instantaneous value for an intermediatecircuit capacitor connected to the energy storage device.

The damping circuit can advantageously be used in a conventionalon-board power supply system. In addition, already existing components,such as a current-limiting inductor, for example, can be used in theconfiguration of the damping circuit, which results in reduced costs,reduced complexity and reduced requirement in terms of installationspace.

In accordance with one embodiment of the damping circuit according tothe invention, the damping circuit can furthermore comprise a bandpassfilter, which is coupled between the current detection device and theclosed-loop control circuit and which is designed to filter frequencycomponents of the output current signal outside a predeterminablefrequency range. This advantageously enables damping, as required, inthe critical resonant frequency range, with the result that no dampingand therefore no unnecessary consumption of resources takes place inuncritical ranges. In particular in frequency ranges in which highpowers would otherwise need to be applied for active damping, butdamping of the current ripple is not necessarily required at all, thiscan result in an increase in the efficiency of the damping circuit.

In accordance with a further embodiment of the damping circuit accordingto the invention, the current detection device can be designed to detecta difference between the output current of the energy storage device andthe output current of a DC voltage intermediate circuit, which isconnected to the energy storage device. This provides the advantage thatit is possible to dispense with a particularly low-induction connectionof a current detection device to the intermediate circuit capacitor.

In accordance with a further embodiment of the damping circuit accordingto the invention, the closed-loop control circuit can have a summingelement, which subtracts the output current signal from the setpointcurrent signal, and a current controller, which generates the currentcontrol signal depending on the output signal of the summing element.

In accordance with a further embodiment of the damping circuit accordingto the invention, the first winding of the transformer can comprise anoutput-side current limitation inductor of the energy storage device.This has the advantage that the current-limiting inductor alreadyprovided in any case can be provided for a dual use in order to save oninstallation space and components.

In accordance with a further embodiment of the damping circuit accordingto the invention, the compensation current generation device can have anH-bridge circuit with in each case two switching devices in each of thebridge branches. This provides the advantage that low-resistancelow-voltage switches can be used for the switching device which can beused as soft-switching full bridge to implement active closed-loopcontrol of damping with very low losses.

In accordance with a further embodiment of the damping circuit accordingto the invention, the compensation current generation device can becoupled to a supply connection of one of the energy storage modules andcan be designed to be fed a supply voltage from the energy storagemodule for generating the compensation current. There is therefore nolonger any need for an external voltage supply, and the damping circuitcan be fed from the energy storage device itself.

In accordance with one embodiment of the system according to theinvention, the system can furthermore have a DC voltage intermediatecircuit, which is coupled to output connections of the energy storagedevice. Advantageously, the system can furthermore comprise an inverter,which is coupled to the DC voltage intermediate circuit, and an electricmachine, which is coupled to the inverter. The inverter can be designedto convert the voltage of the DC voltage intermediate circuit into aninput voltage for the electric machine. This is particularlyadvantageous since the system comprising the inverter and the electricmachine, as variable current source, can feed back currents withfrequency-dependent fluctuations into the energy storage device via theDC voltage intermediate circuit. These frequency-dependent fluctuationscan be damped particularly effectively via the damping circuit.

Further features and advantages of embodiments of the invention aregiven in the description below with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematic illustration of a system comprising an energystorage device and a damping circuit in accordance with one embodimentof the present invention;

FIG. 2 shows a schematic illustration of an exemplary embodiment of anenergy storage module of an energy storage device in accordance with afurther embodiment of the present invention;

FIG. 3 shows a schematic illustration of a further exemplary embodimentof an energy storage module of an energy storage device in accordancewith a further embodiment of the present invention;

FIG. 4 shows a schematic illustration of a system comprising an energystorage device and a damping circuit in accordance with a furtherembodiment of the present invention; and

FIG. 5 shows a schematic illustration of a method for dampingoscillations of the output current of an energy storage device inaccordance with a further embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a system which comprises an energy storage device 10 forproviding a supply voltage by energy storage modules 3 connected inseries in an energy supply string between two output connections 1 a, lbof the energy storage device 10. The energy storage device 10 canalternatively also have a plurality of energy supply strings connectedin parallel. The energy storage device 10 acts as current source of avariable output current owing to the actuation of the energy storagemodules 3.

The energy storage device 10 can in this case be coupled to an inputconnection of a DC voltage intermediate circuit 9 a via a storageinductance 2 a at the output connection 1 a of the energy storage device10. The storage inductance 2 a can be implemented, for example, by aconcentrated component, such as a current-limiting inductor, or aplurality of distributed components. Alternatively, parasiticinductances of the energy storage device 10 can also be used as storageinductance 2 a. By correspondingly activating the energy storage device10, the current flow into the DC voltage intermediate circuit 9 a can becontrolled. If the mean voltage upstream of the storage inductance 2 ais higher than the instantaneous intermediate circuit voltage, a currentflow into the DC voltage intermediate circuit 9 a takes place, and if,on the other hand, the mean voltage upstream of the storage inductance 2a is lower than the instantaneous intermediate circuit voltage, acurrent flow into the energy storage device 10 takes place. The maximumcurrent is in this case limited by the storage inductance 2 a ininteraction with the DC voltage intermediate circuit 9 a.

The energy storage device 10 has at least two energy storage modules 3connected in series in an energy supply string. By way of example, thenumber of energy storage modules 3 in FIG. 1 is two, but any othernumber of energy storage modules 3 is likewise possible. The energystorage modules 3 each have two output connections 3 a and 3 b, viawhich a module output voltage of the energy storage modules 3 can beprovided. The module output voltages of the energy storage modules 3 canbe added selectively to give the total output voltage of the energystorage device 10.

Exemplary designs of the energy storage modules 3 are shown in FIGS. 2and 3 in greater detail. The energy storage modules 3 each comprise acoupling device 7 with a plurality of coupling elements 7 a and 7 c andpossibly 7 b and 7 d. The energy storage modules 3 furthermore comprisein each case one energy storage cell module 5 having one or more energystorage cells 5 a, 5 k connected in series.

The energy storage cell module 5 can in this case have, for example,batteries 5 a to 5 k connected in series, for example lithium-ionbatteries or rechargeable lithium-ion batteries. In this case, thenumber of energy storage cells 5 a to 5 k in the energy storage module 3shown in FIG. 2 is two, for example, but any other number of energystorage cells 5 a to 5 k is likewise possible.

The energy storage cell modules 5 are connected to input connections ofthe associated coupling device 7 via connecting lines. The couplingdevice 7 is in the form of a full-bridge circuit with in each case twocoupling elements 7 a, 7 c and two coupling elements 7 b, 7 d, by way ofexample in FIG. 2. The coupling elements 7 a, 7 b, 7 c, 7 d can in thiscase each have an active switching element, for example a semiconductorswitches, and a freewheeling diode connected in parallel therewith. Thesemiconductor switches can comprise, for example, field-effecttransistors (FETs). In this case, the freewheeling diodes can also eachbe integrated in the semiconductor switches.

The coupling elements 7 a, 7 b, 7 c, 7 d in FIG. 2 can be actuated insuch a way that the energy storage cell module 5 is connectedselectively between the output connections 3 a and 3 b or such that theenergy storage cell module 5 is bridged. By suitably actuating thecoupling devices 7, therefore, individual energy storage cell modules 5of the energy storage modules 3 can be integrated in the series circuitof an energy supply string in a targeted manner.

With reference to FIG. 2, the energy storage cell module 5 can beconnected, for example, in the forward direction between the outputconnections 3 a and 3 b by virtue of the active switching element of thecoupling element 7 d and the active switching element of the couplingelement 7 a being set to a closed state while the other two activeswitching elements of the coupling elements 7 b and 7 c are set to anopen state. In this case, the voltage U_(M) is present between theoutput terminals 3 a and 3 b of the coupling device 7. A bridging statecan be set, for example, by virtue of the two active switching elementsof the coupling elements 7 a and 7 b being set to the closed state whilethe two active switching elements of the coupling elements 7 c and 7 dare kept in the open state. A second bridging state can be set, forexample, by the two active switches of the coupling elements 7 c and 7 dbeing set to the closed state while the active switching elements of thecoupling elements 7 a and 7 b are kept in the open state. In bothbridging states, the voltage 0 is present between the two outputterminals 3 a and 3 b of the coupling device 7. Likewise, the energystorage cell module 5 can be connected in reverse between the outputconnections 3 a and 3 b of the coupling device 7 by virtue of the activeswitching elements of the coupling elements 7 b and 7 c being set to theclosed state while the active switching elements of the couplingelements 7 a and 7 d are set to the open state. In this case, thevoltage −U_(M) is present between the two output terminals 3 a and 3 bof the coupling device 7.

The total output voltage of the energy supply string can in this case beset in each case stepwise, wherein the number of steps scales with thenumber of energy storage modules 3. In the case of a number of n firstand second energy storage modules 3, the total output voltage of theenergy supply string can be adjusted in 2n+1 steps between −n·U_(M), . .. , 0, . . . , +n·U_(M).

FIG. 3 shows a further exemplary embodiment of an energy storage module3. The energy storage module 3 shown in FIG. 3 differs from the energystorage module 3 shown in FIG. 2 only in that the coupling device 7 hastwo instead of four coupling elements, which are connected to oneanother in a half-bridge circuit instead of in a full-bridge circuit.

In the variant embodiments illustrated, the active switching elementscan be in the form of power semiconductor switches, for example in theform of IGBTs (insulated gate bipolar transistors), JFETs (junctionfield-effect transistors) or else MOSFETs (metal oxide semiconductorfield-effect transistors).

By virtue of the coupling elements 7 a, 7 b, 7 c, 7 d, the outputvoltage of the energy supply string can be varied in steps from anegative maximum value up to a positive maximum value via suitableactuation. The graduation of the voltage level in this case resultsdepending on the graduation of the individual energy storage cellmodules 5. In order to obtain, for example, a mean voltage value betweentwo voltage levels predetermined by the graduation of the energy storagecell modules 5, the coupling elements 7 a, 7 b, 7 c, 7 d of an energystorage module 3 can be actuated in clocked fashion, for example withpulse width modulation (PWM), with the result that the affected energystorage module 3 produces, when averaged over time, a module voltagewhich can have a value of between zero and the maximum possible modulevoltage determined by the energy storage cells 5 a to 5 k. The actuationof the coupling elements 7 a, 7 b, 7 c, 7 d can in this case beperformed, for example, using a control device, which is designed toperform, for example, closed-loop control of the current withsubordinate open-loop control of the voltage, with the result thatgraduated connection or disconnection of individual energy storagemodules 3 can take place.

The system in FIG. 1 comprises, in addition to the energy storage device10, also an inverter 12 and an electric machine 13. By way of example,the system in FIG. 1 is used for feeding a three-phase electric machine13. However, provision can also be made for the energy storage device 1to be used for generating electrical current for an energy supplysystem. Alternatively, the electric machine 13 can also be a synchronousor asynchronous machine, a reluctance machine or a brushless DC motor(BLDC). In this case, it can also be possible to use the energy storagedevice 10 in stationary systems, for example in power stations, inelectrical energy generation plants, such as, for example, windturbines, photovoltaic installations or combined heat and powergeneration plants, in energy storage installations such as compressedair storage power stations, battery storage power stations, flywheelenergy stores, pumped-storage facilities or similar systems. A furtheruse possibility for the system in FIG. 1 consists in passenger or goodstransport vehicles which are designed for locomotion on or beneath thewater, for example ships, motor boats or the like.

In the exemplary embodiment in FIG. 1, the DC voltage intermediatecircuit 9 a feeds a pulse-controlled inverter 12, which provides athree-phase AC voltage for the electric machine 13 from the DC voltageof the DC voltage intermediate circuit 9 a. However, any other type ofconverter can also be used for the inverter 12, depending on therequired voltage supply for the electric machine 13, for example a DCvoltage converter. The inverter 12 can be operated, for example, usingspace vector pulse width modulation (SVPWM).

By virtue of the actuation of the inverter 12 and the alternating powerconsumption of the electric machine 13, the composite structurecomprising the inverter and the machine acts as a variable currentsource, with respect to the DC voltage intermediate circuit 9 a, whichvariable current source can excite the resonance circuit comprising theenergy storage device 10, the storage inductance 2 a and the DC voltageintermediate circuit 9 a to resonance. The resonant frequency of thisresonance circuit is dependent, inter alia, on the number of connectedenergy storage modules 3, the instantaneous values for the intermediatecircuit voltage and the energy content of the storage inductance. Inaddition, the resonant frequency can be subject to relatively long-termfluctuations which can be dependent, for example, on the componenttolerance, the aging of the components, the temperature and furtherinfluences. In order to reduce these oscillations and therefore theripple of the current towards and away from the DC voltage intermediatecircuit 9 a, it is necessary to take measures which can be used to dampthese resonances or the current ripple.

For this, a current detection device 8 is provided in the system in FIG.1 as part of a damping circuit, which current detection device isdesigned to detect an output current of the energy storage device 10 andto generate an output current signal which is dependent on the outputcurrent. The current detection device 8 can have, for example, a firstcurrent sensor 8 a, which detects a current flowing from the DC voltageintermediate circuit 9 a into the energy storage device 10 or a currentflowing from the energy storage device 10 into the DC voltageintermediate circuit 9 a. The current detection device 8 can furthermorehave a second current sensor 8 b, for example, which detects a currentflowing from the DC voltage intermediate circuit 9 a into the variablecurrent source 14 or a current flowing from the variable current source14 into the DC voltage intermediate circuit 9 a. The detected currentscan be detected, for example, as the difference between the outputcurrent of the energy storage device 10 and the output current of a DCvoltage intermediate circuit 9 a connected to the energy storage device10. For this, a summing element 8 c can be provided in the currentdetection device 8, which summing element subtracts the currentsdetected by the first and second current sensors 8 a and 8 b from oneanother.

It may also be possible to design the current detection device 8 in sucha way that an output current signal is detected within the energystorage device 10 with a plurality of energy supply strings on each ofthe energy supply strings. This can make it possible to adjust each ofthe energy supply strings separately, in particular since each of theenergy supply strings forms, with the DC voltage intermediate circuit 9a, a dedicated resonant circuit.

The system further comprises, as part of the damping circuit, aclosed-loop control circuit 6, which is coupled to the current detectiondevice 8 and which is designed to adjust, by closed-loop control, theoutput current signal to a setpoint current signal 6 c and to output acorresponding current control signal. The closed-loop control circuit 6can have a summing element 6 b for this purpose, which subtracts theoutput current signal from the setpoint current signal 6 c. With the aidof a current controller 6 a, the current control signal can be generateddepending on the output signal of the summing element 6 b. The setpointcurrent signal 6 c can be zero, for example. Alternatively, any otherdesired value can also be predetermined for the setpoint current signal6 c.

The current control signal can be fed into a compensation currentgeneration device 4, which is coupled to the closed-loop control circuit6, and which is designed to generate a compensation current depending onthe current control signal, which compensation current compensates forfluctuations in the output current of the energy storage device 10. Thiscompensation current can, for example, feed a transformer 2 formed fromthe storage inductance 2 a as first winding and a second winding 2 b.The first winding 2 a can in this case be galvanically isolated from thesecond winding 2 b.

In the example in FIG. 1, the compensation current generation device 4can have a voltage source 4 a which can be adjusted by the currentcontroller 6 a. The voltage source 4 a in this case feeds the secondwinding 2 b of the transformer 2. It may be possible, for example, tointegrate the storage inductance 2 a as magnetization inductance of thetransformer 2.

Optionally, a bandpass filter 11 can be provided, which is coupledbetween the current detection device 8 and the closed-loop controlcircuit 6 and which is designed to filter frequency components of theoutput current signal outside a predeterminable frequency range. Thisprevents, for example, interventions in respect of active damping infrequency ranges in which high powers would be required for adjustingthe current ripple but no or no considerable reduction in the currentripple is required.

FIG. 4 shows a schematic illustration of a further system comprising anenergy storage device 10 and a damping circuit. The system in FIG. 4differs from the system in FIG. 1 substantially in that the compensationcurrent generation device 4 has an H-bridge circuit with in each casetwo switching devices 4 b, 4 c, 4 d, 4 e in each of the bridge branches.The switching devices 4 b, 4 c, 4 d, 4 e can comprise low-voltageswitches, for example. The switching devices 4 b, 4 c, 4 d, 4 e can forexample be in the form of power semiconductor switches, for example inthe form of IGBTs (insulated gate bipolar transistors), JFETs (junctionfield-effect transistors) or MOSFETs (metal oxide semiconductorfield-effect transistors).

Furthermore, the compensation current generation device 4 can be coupledto a supply connection 3 c of one of the energy storage modules 3. Asupply voltage can be provided from the energy storage cell module 5 viathe supply connection 3 c of the energy storage module 3, which supplyvoltage can be used to feed the compensation current generation device4. By implementing the compensation current generation device 4 as afull bridge with soft-switching switching devices 4 b, 4 c, 4 d, 4 e,low-loss generation of the compensation current for feeding into thesecond winding 2 b of the transformer can be realized.

FIG. 5 shows a schematic illustration of an exemplary method 20 fordamping oscillations of the output current of an energy storage device,in particular an energy storage device 10, as is explained in connectionwith FIGS. 1 to 4. The method 20 can use, for example, a damping circuitas illustrated in FIGS. 1 and 4 for this.

The method 20 for damping oscillations of the output current of anenergy storage device 10 comprises, in a first step 21, detection of anoutput current of the energy storage device. In a second step 22,generation of an output current signal which is dependent on thedetected output current is performed. In a third step 23, adjustment, byclosed-loop control, of the output current signal to a setpoint currentsignal is performed. Then, in a fourth step 24, outputting of a currentcontrol signal corresponding to the closed-loop control can beperformed, whereupon, on this basis, in a step 25, generation of acompensation current can take place, which compensates for fluctuationsin the output current of the energy storage device 10. Finally, in step26, feeding of the compensation current into the second winding 2 b of atransformer 2, which has a first winding 2 a which is coupled to anoutput connection 1 a of the energy storage device 10 and a secondwinding 2 b, which is galvanically isolated from the first winding 2 a,is performed.

What is claimed is:
 1. A damping circuit for an energy storage devicewhich comprises one or more energy storage modules connected in seriesin one or more energy supply strings, said energy storage modules havingat least one energy storage cell and a coupling device having amultiplicity of coupling elements, which coupling device is designed toconnect the energy storage cell selectively into the respective energysupply string or to bridge said energy storage cell, the damping circuitcomprising: a current detection device, which is designed to detect anoutput current of the energy supply strings or the energy storage deviceand to generate an output current signal dependent on the outputcurrent; a closed-loop control circuit, which is coupled to the currentdetection device and is designed to adjust the output current signal toa setpoint current signal and to output a corresponding current controlsignal; a transformer, which has a first winding, which is coupled to anoutput connection of the energy storage device, and a second windingwhich is galvanically isolated from the first winding; and acompensation current generation device, which is coupled to theclosed-loop control circuit and is designed to feed a compensationcurrent, which compensates for fluctuations in the output current of theenergy storage device, into the second winding of the transformerdepending on the current control signal.
 2. The damping circuitaccording to claim 1, further comprising: a bandpass filter, which iscoupled between the current detection device and the closed-loop controlcircuit and which is designed to filter frequency components of theoutput current signal outside a predeterminable frequency range.
 3. Thedamping circuit according to claim 1, wherein the current detectiondevice is designed to detect a difference between the output current ofthe energy storage device and the output current of a DC voltageintermediate circuit, which is connected to the energy storage device.4. The damping circuit according to claim 1, wherein the closed-loopcontrol circuit has a summing element, which subtracts the outputcurrent signal from the setpoint current signal, and a currentcontroller, which generates the current control signal depending on theoutput signal of the summing element.
 5. The damping circuit accordingto claim 1, wherein the first winding of the transformer comprises anoutput-side current limitation inductor of the energy storage device. 6.The damping circuit according to claim 1, wherein the compensationcurrent generation device has an H-bridge circuit within each case twoswitching devices in each of the bridge branches.
 7. The damping circuitaccording to claim 6, wherein the compensation current generation deviceis coupled to a supply connection of one of the energy storage modulesand is designed to be fed a supply voltage from the energy storagemodule for generating the compensation current.
 8. A system, comprising:an energy storage device, which has one or more energy storage modules,which are connected in series in one or more energy supply strings, saidenergy storage modules comprising at least one energy storage cell and acoupling device having a multiplicity of coupling elements, whichcoupling device is designed to connect the energy storage cellselectively into the respective energy supply string or to bridge saidenergy storage cell; and a damping circuit according to claim
 1. 9. Thesystem according to claim 8, further comprising: a DC voltageintermediate circuit, which is coupled to output connections of theenergy storage device.
 10. The system according to claim 9, furthercomprising: an inverter, which is coupled to the DC voltage intermediatecircuit; and an electric machine, which is coupled to the inverter,wherein the inverter is designed to convert the voltage of the DCvoltage intermediate circuit into an input voltage for the electricmachine.
 11. A method for damping oscillations of the output current ofan energy storage device, which has one or more energy storage modules,which are connected in series in an energy supply string, said energystorage modules comprising at least one energy storage cell and acoupling device having a multiplicity of coupling elements, whichcoupling device is designed to connect the energy storage cellselectively into the respective energy supply string or to bridge saidenergy storage cell, said method comprising the following steps:detecting an output current of the energy supply strings or the energystorage device; generating an output current signal which is dependenton the detected output current; applying closed-loop control to theoutput current signal to adjust it to a setpoint current signal;outputting a current control signal corresponding to the closed-loopcontrol; generating a compensation current, which compensates forfluctuations in the output current of the energy storage device; andfeeding the compensation current into the second winding of atransformer, which has a first winding, which is coupled to an outputconnection of the energy storage device, and a second winding, which isgalvanically isolated from the first winding.